Electronic alarm systems for homes and offices often consist of several separate components which have a working relationship with each other. Thus signal devices assigned to different locations can send a signal to a central switching device which then can activitate one or several alarm devices.
In present systems of this sort the working relationship of such components is carried out through the general electrical wiring system so that the laying of special cables is unnecessary. With the increasing number and distance of the areas that must be guarded, this is an essential advantage.
In such alarm systems, the components that must be brought into a working relationship with each other are simply connected into the outlet at their place of location, through which they send out or from which they receive special signals.
These signals could be, for example, sound frequencies or impulses, which are transmitted to the wiring of a suitable coupling device. Naturally the outlet can also be used as an energy source so that it can always replenish an accumulator joined to the special component so that the system remains functioning even when the electicity is shut off.
In present systems, binary coded signals, i.e. pulse trains, are transmitted so that for example the alarm signal from a certain signal device can be identified. In this case, a certain frequency is assigned to one of the two logical conditions, usually "1", whereas the absence of this frequency represents the logical condition "0".
Practical experience with such systems shows that the desired reliability of signal transmission is especially not achieved if the binary signals are transmitted through a wiring system with high interference. Such interference can for example be caused by motors, phase gate power controls, fluorescent lamps, etc. and are always present on usual electric supply wiring systems.